Ultra high-speed optical fiber transmission generally uses a wavelength division multiplexing (WDM)transmission technique that transmits multiple optical signals with different wavelengths in a bundle in order to effectively use a wavelength range (or frequency bandwidth) available for the signal transmission, splits the signals in accordance with the original wavelengths at a receiving terminal after transmission through optical fiber, and then allows optical receivers to receive the signals. In addition, a polarization multiplexing technique is also examined for more effective use of a frequency bandwidth.
The polarization multiplexing method uses differences in polarization states of the light. A transmission side modulates two sets of optical signals using an independent information signal. The optical signals are converted into polarization states orthogonal to each other, multiplexed, and then transmitted through optical fiber. The optical signal polarization state can be expressed as a given point on a Poincare sphere. The optical signal polarization state is subject to random transformation in an optical fiber transmission path but maintains orthogonality of the two polarization states. Accordingly, by performing conversion of the polarization state and polarization splitting, a receiving side can split two multiplexed original optical signals. The same wavelength interval can be used to transmit the double quantity of information.
Several methods are known to implement the polarization-multiplexed transmission, specifically, polarization multiplexing on a transmitter and polarization splitting on a receiver.
The following describes a coherent polarization-multiplexed transmission system as one of technologies of the related art for polarization-multiplexed optical receivers. An example of the system is published in: P. J. Winzer, “Spectrally Efficient Long-Haul Optical Networking Using 112-Gb/s polarization-multiplexed 16-QAM”, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 28, NO. 4, FEB. 15, 2010, pp. 547-556 (non-patent document 1). FIG. 1 illustrates a digital coherent polarization-multiplexed transmission system of the related art using a polarization-multiplexed optical transmitter and a polarization-diversity coherent optical receiver.
In a polarization-multiplexed optical transmitter 100, a laser source 101 generates an unmodulated laser beam. An optical splitter 108 splits the unmodulated laser beam into two beams and supplies them to two IQ optical field modulators 102-1 and 102-2. The IQ optical field modulator (also referred to as an IQ modulator) includes two sets of MZ modulators that are each parallel placed on a lithium niobate substrate. The IQ optical field modulator applies a fast modulated voltage signal to a modulation signal input terminal of each MZ modulator and is thereby capable of independently modulating in-phase (I) components (or real part) and quadrature-phase (Q) components (or imaginal part) of an optical field. According to this example, an independent multilevel information signal is input to each of input terminals 103-1 and 103-2 corresponding to in-phase modulation signals and input terminals 104-1 and 104-2 corresponding to quadrature-phase modulation signals for the two IQ optical field modulators. As a result, each of the MZ modulators outputs independent multilevel modulation light modulated on a two-dimensional complex plane. The light rays are converted into an X-polarization optical modulation signal 105 and a Y-polarization optical modulation signal 106 so that the polarization states are orthogonal to each other. The signals are input to a polarization multiplexer 107. The polarization multiplexer 107 then outputs a polarization-multiplexed optical transmission signal 120.
FIGS. 2A and 2B illustrate a method of displaying the multilevel signals, signal points according to optical multilevel modulation, and polarization-multiplexed transmission.
FIG. 2A illustrates a signal point on the complex plane (IQ plane). Modulation states of multilevel signals can be represented using optical fields at the center timing (decision time) for waveform of modulated signal. A signal point can be represented using complex Cartesian coordinates (IQ coordinates) or polar coordinates that use amplitude r (n) and phase φ(n).
FIG. 2B shows a quarternary phase shift keying (QPSK) signal that transmits 2-bit information (00, 01, 11, and 10) per symbol using four values (π/4, 3π/4, −3π/4, and −π/4) as phase angle φ(n). The signal can be generated as follows, for example. A two-value electric signal is input to each of two input terminals for the IQ optical field modulator 102-1, that is, the input terminal 103-1 for in-phase modulation signals and the input terminal 104-1 for quadrature-phase modulation signals. An in-phase component (I) and a quadrature-phase component (Q) are modulated to a positive two-value signal and a negative two-value signal, respectively. Polarization-multiplexed 100-gigabit transmission uses the quarternary phase shift keying modulation for polarized optical signals. The modulation uses an electric two-value signal approximately at 28 Gbps. The signal is used to generate a quarternary phase shift keying signal capable of transmitting 2-bit information per symbol. The transmission rate is 56 Gbps.
FIG. 2C shows 16 quadrature amplitude modulation (16QAM) capable of higher information transmission efficiency. According to 16QAM, signal points are arranged in a grid. One symbol can transmit four bits of information. In the example of FIG. 2C, Q-axis coordinates represent values for two high-order bits (10xx, 11xx, 01xx, and 00xx). I-axis coordinates represent values for two low-order bits (xx10, xx11, xx01, and xx00). Such a multilevel signal can be generated by supplying a multilevel electric signal (four values in this example) to each of the input terminal 103-1 for in-phase modulation signals and the input terminal 104-1 for quadrature-phase modulation signals in FIG. 1 and specifying electric field coordinates (i(t), q(t)) for in-phase components and quadrature-phase components. At this time, the X-polarization optical modulation signal 105 is output and its electric field is expressed as (i(t)+jq(t))exp(jωt). In the expression, ω signifies an optical angle frequency of the laser source 101 and j signifies an imaginary unit. To generate an intricate optical field signal, an ultra high-speed DA converter may be used to generate voltage signals corresponding to the real part i(t) and the imaginal part q(t) of a complex electric field signal. The voltage signals may be then applied to the input terminal 103-1 for in-phase modulation signals and the input terminal 104-1 for quadrature-phase modulation signals.
FIG. 2D is a conceptual diagram of polarization multiplexing. The light wave is a type of electromagnetic waves. There are two independent orthogonal polarization states (e.g., horizontal polarization and vertical polarization) depending on an oscillation direction of the electric field in relation to the traveling direction. Different information signals can be used to modulate, multiplex, and transmit two optical field components (X-polarization component and Y-polarization component in FIG. 2D). The above-mentioned 100-gigabit transmission multiplexes the X-polarization and the Y-polarization each with an independent quarternary phase shift keying signal at 56 Gbps. The total transmission rate is 112 Gbps. The transmission rate exceeds 100 Gbps due to encoding or addition of error correction information. The transmission rate might vary with transmission systems.
As described with reference to FIG. 1, the polarization-multiplexed optical transmission signal 120 is transmitted over an optical fiber transmission path 121 as long as several tens to thousands of kilometers. At the same time, the signal 120 is subject to transmission degradation due to wavelength dispersion of the optical fiber. A polarization diversity coherent optical receiver 110 of the related art then receives the signal 120. According to the coherent reception technique, light is output from a local oscillator 116 included in the receiver and is used as the reference to detect electric field components in an optical signal. A polarization splitting and optical 90-deg. hybrid circuit 113 splits a polarization-multiplexed received optical signal 122 into eight components, that is, four S-polarization components 123 and four P-polarization components 124. Four balanced optical receivers 111-1, 111-2, 111-3, and 111-4 receive the eight components.
The local oscillator 116 included in the receiver is set to approximately the same optical frequency as the received optical signal 122. The output light is connected to another input port of the polarization splitting and optical 90-deg. hybrid circuit 113 and is split to the balanced optical receivers 111-1, 111-2, 111-3, and 111-4 similarly to the signal light. The input signal light interferes with the local light in each balanced optical receiver and is converted into an electric signal. The output high-speed electric signals are sampled in AD converters 112-1, 112-2, 112-3, and 112-4 and are converted into digital signals. A digital signal processing circuit 114 separates polarization components from the input digital signals and demodulates the signals to output an X-polarization component 125 and a Y-polarization component 126 that are originally contained and are now demodulated. A multilevel signal decision circuit 115 decodes these signals to output an information signal 127. Generally, the receiver (transponder) is followed by a framer and error correction circuit 128. The framer and error correction circuit 128 analyzes a received signal and detects the beginning of a data frame in the signal. The framer and error correction circuit 128 also performs error correction on the received signal using predetermined error correction information supplied before the transmission, reads information from the header, and processes channels and monitor information. The framer and error correction circuit 128 according to the configuration also checks if the polarization splitting process is successful. The framer and error correction circuit 128 issues a reset signal 129 to the digital signal processing circuit 114 so as to re-execute the process if it is determined that the polarization splitting process is unsuccessful.
FIG. 3A is an explanatory diagram of the digital signal processing circuit 114 in the digital coherent polarization-multiplexed transmission system of the related art and illustrates signal processing inside the circuit. The circuit is supplied with four electric digital signals, that is, an S-polarization in-phase component (SI), an S-polarization quadrature-phase component (SQ), a P-polarization in-phase component (PI), and a P-polarization quadrature-phase component (PQ). The signals are each equivalent to a digital signal sequence that is sampled at a rate equal to or twice the symbol rate of the modulation signal and provides the resolution of five to eight bits. A 100G coherent receiver reaches up to 56 giga-samples per second as a sampling rate. Some to several hundreds of input digital signal sequences and internal arithmetic processes are parallel processed in accordance with LSI operation speeds. The following description supposes a complex signal S having real part SI and imaginal part SQ and a complex signal P having real part PI and imaginal part PQ. Wavelength dispersion compensation circuits 130-1 and 130-2 compensate the input signals S and P for an effect of wavelength dispersion over the transmission path. Retiming circuits 131-1 and 131-2 perform timing correction and sampling on the signals so that the waveform central time matches any of odd-numbered or even-numbered sampling times. A butterfly FIR filter 132 is equivalent to a digital equalization filter that adaptively corrects waveforms. The functions include elimination of inter-symbol interference, compensation for residual chromatic dispersion, and polarization splitting as described in this specification. The butterfly FIR filter 132 functions as a two-input and two-output filter that uses two sets of S-polarization and P-polarization complex signals as input and outputs two sets of complex signals having original X-polarization component and Y-polarization component multiplexed in the transmission side. As shown in FIG. 3A, the butterfly FIR filter 132 includes four complex FIR filters, that is, an FIR filter (HXX) 133, an FIR filter (HYY) 134, an FIR filter (HXY) 135, and an FIR filter (HYX) 136. Adder circuits 137-1 and 137-2 synthesize output signals to generate two sets of output signals x and y.
FIG. 3B shows an internal configuration of the complex FIR filter. In FIG. 3B, an input complex signal sequence 144 is equivalent to a complex digital signal having about one or two sampling points per symbol. Inside the FIR filter, a ladder-type digital filter is configured using a delay circuit 146 having the delay time (Z^−1) equal to a time interval between sampling points, a complex tap multiplier circuit 147, and a complex adder circuit 148. The filter multiplies a complex tap weight wi by a sample value corresponding to the sample time at every sample time, adds the results to each other, and sequentially calculates and outputs an output complex signal sequence 145. A compensation data setting signal 149 sets the complex tap weights so as to minimize the distortion of the output complex signal sequence 145.
In FIG. 3A, the butterfly FIR filter 132 inputs part of two output signals to CMA arithmetic circuits 140-1 and 140-2 and calculates error signals (EX,CMA, EY,CMA). Tap optimization circuits 143-1 and 143-2 perform adaptive equalization and sequentially update tap coefficients for the FIR filters so as to nullify the error signals. The CMA (Constant Modulus Algorithm) is used for signal processing. A CMA circuit calculates an amplitude error (e.g., deviation from a specified value 1.0) in the output signal and outputs the result as an error signal. When receiving a polarization-multiplexed QPSK signal like the 100G signal as described above, it is known that an error signal is nullified if one of the polarization components is completely separated and the waveform distortion is corrected. If the CMA circuit operates normally, the butterfly filter generates an output signal that is compensated for waveform distortion and undergoes polarization splitting.
A frequency and phase correction circuit 141 corrects a frequency difference (frequency offset) between the local oscillator and the transmission light source and corrects an output signal phase shift (shift in the rotation direction on the complex plane). The circuit outputs demodulated signal sequences X and Y. Non-patent document 1 reports that the above-mentioned CMA computation does not function completely because a high-order multilevel signal contains amplitude modulation such as the 16-value modulation inherently causes unstable amplitude. To solve this problem, the following decision-directed method is also used. That is, the CMA computation is used only for initial convergence of tap coefficients for the butterfly FIR filter 132. Decision error calculation circuits 142-1 and 142-2 detect errors between the arranged signal points corresponding to output signals after the tap coefficients are converged to some extent and the multilevel demodulation becomes available. Tap optimization circuits 143-1 and 143-2 are supplied with the detected errors and optimize the taps so as to minimize the errors. The tap optimization circuits 143-1 and 143-2 are provided with reset signal input terminals 139-1 and 139-2 that enable to start reconvergence from the outside as needed.
FIG. 4 is an explanatory diagram showing another polarization-multiplexed transmission system of the related art and illustrates the system that provides the reception side with an automatic optical polarization controller for polarization splitting. A polarization-multiplexed optical transmitter 150 of the related art uses two optical MZ modulators 151-1 and 151-2 for optical signal modulation. A high-speed two-value or multilevel analog electric signal is applied to each of the optical MZ modulators 151-1 and 151-2 for corresponding modulation signals. With respect to the optical MZ modulator 151-2, a low-speed sine wave oscillator 153 outputs a small-amplitude sine wave signal at frequency f. The sine wave signal is applied to a bias signal input terminal 154 in order to optimize a modulated bias voltage. FIG. 4 omits a direct current voltage source for applying a bias and the detailed illustration about the optical MZ modulator 151-1. To optimize the bias voltage, for example, a low-speed optical detector detects an intensity change in the output optical waveform immediately after the optical MZ modulator 151-2. The optical detector adjusts direct current voltage components of the bias signal so as to maximize the components of the frequency f.
Generally, it is known that superposing such small-amplitude modulation degrades the transmission signal quality. A limited modulation bandwidth is used for the bias signal input terminal 154 as a bias modulation terminal. For this reason, the above-mentioned sine wave signal generally uses a low frequency (f=some to several tens of kilohertz) and is set to a modulation degree of 1% to 5%.
Output lights from the two optical MZ modulators 151-1 and 151-2 are converted into polarization states X and Y orthogonal to each other. A polarization multiplexing circuit 107 applies polarization multiplexing to the states and outputs the optical signal. Such a polarization multiplexing circuit uses a polarization beam combiner, a polarization optical splitter, or an optical coupler without polarization dependency. The optical signal is transmitted through the optical fiber transmission path 121. A polarization splitting optical receiver 160 of the related art receives the optical signal. An input portion is provided with an automatic optical polarization controller 158 and a polarization splitting circuit 159 that splits the received signal into two polarization components X and Y.
There are some control methods proposed for the automatic optical polarization controller 158 according to the above-mentioned configuration. The example here assumes the use of low-speed bias modulation components supplied from the transmission side. A low-speed optical detector 155 is provided at a Y-component reception port and branches part of the received light. A bandpass filter extracts components that match the frequency f. A maximization control circuit 157 controls the automatic optical polarization controller 158 so as to maximize the component of the frequency f and thereby provides automatic control so as to always output the Y-polarization component from the transmission side to a Y-component reception port on the reception side. This configuration takes effect when the bias modulation on the transmission side generates an intensity modulation component of the frequency f in the transmitted Y-polarization components. In some cases, it may be necessary to maximize a component of frequency 2f instead depending on optical signal modulation systems (e.g., two-value phase modulation). An optical receiver 162-2 can always maintain an optimal reception state capable of maximizing the Y-polarization component even if the polarization control changes a polarization major axis state of the optical fiber transmission path 121.
Another polarization splitting system is proposed. The system measures reception quality inside the receiver instead of detecting bias components as described above. The system outputs the reception quality as a reception quality signal 161 to the outside and controls the automatic optical polarization controller 158 so as to maximize a value of the signal. The reception quality signal is available if its quantity is sufficient to reflect the signal quality with high sensitivity. For example, the reception quality signal is available as eye opening of a received signal, sign-inverted EVM (Error Vector Magnitude) indicative of the signal point dispersion, negative logarithmic value for a bit error rate, or Q value.